I had this once with my iPhone and Mac book
I cant remember the exact fix and I can't recreate the issue but it came down to a difference between time zones on my Mac and iPhone.
I use mobile me and I think the issue was with a time zone in there come to think of it.
If you ise mobile me then check there, if you use OWA some other company sync system then check those settings.
Sorry this is vague but it was a year ago I had the issue and I remember there being no help on the net.
Your problem could be totally different of course, in which case Ive no idea
Si